Production of isoprene in the United States of America amounts to approximately 200 million pounds per year, and world production amounts to approximately 2 billion pounds per year. Isoprene is used primarily in making cis-1,4-polyisoprene, a synthetic substitute for natural rubber. Polyisoprene and natural rubber are prime components of radial tires, whereas bias tires use other rubbers. Since radials are replacing bias tires, the demand for polyisoprene/natural rubber is increasing. Depending on the supply of natural rubber, more polyisoprene will be needed in the future.
At present, in the United States, all commercial production of isoprene is by extraction from C.sub.5+ streams of olefin plants. The availability of isoprene from this source, however, depends on the output of ethylene and on the feedstock fed to the olefin plants. Heavier feedstocks such as naphtha produce more isoprene while lighter feedstocks such as liquified petroleum gas produce less. As the cracking stocks of olefin plants have become lighter in the recent past, the amount of isoprene produced has declined.
In Eastern Block countries, where ethylene is not produced in such large amounts as in the United States, isoprene is made in dedicated facilities. The Soviet Union is the premier isoprene manufacturer with over 1 billion pounds of capacity. The main synthetic route used is the Prins reaction of isobutene with formaldehyde followed by dehydration of the intermediate 4,4-dimethylmetadioxane (4,4-DMD) adduct.
Other processes available for the production of isoprene include, for example, the reaction of acetylene and acetone, followed by hydrogenation, to give dimethylvinylcarbinol, which is then dehydrated to yield isoprene.
A new and potentially very economical process for the production of isoprene is the dehydration of 2-methylbutyraldehyde (2-MBA). 2-MBA can be produced by hydroformylation of butene and so is an inexpensive raw material which can be manufactured in the large quantities needed for large scale commercial production of isoprene. The dehydration of 2-MBA to isoprene is catalyzed by acids and can be effected with reasonable selectivity. However, since isoprene itself is polymerized by strong acids such as sulfuric acid, such strong acids are not useful as isoprene catalysts.
A wide variety of catalysts, including both molecular sieves and various other materials, have been proposed for use in the aforementioned reactions for the production of isoprene and in chemically-similar reactions. For example, British Pat. No. 673,547 describes a process for the conversion of 1-methoxy-3-methylbut-4-ene to isoprene using a silicate of aluminum as the catalyst.
U.S. Pat. No. 3,253,051 to Yanagita et al. describes a process for the production of isoprene from a mixture of isobutylene and formaldehyde in which the catalyst employed is a mixture of an oxide or hydroxide of a metal such as iron with phosphoric acid, this mixture being supported on a suitable carrier, for example silica or alumina.
U.S. Pat. Nos. 3,872,216 and 3,846,338, both to Kachalova et al., describe methods for producing isoprene from a dioxane employing a tricalcium phosphate catalyst.
British Pat. No. 1,385,348 describes the production of a diene from the corresponding aldehyde using an acidic dehydration catalyst, which can comprise inorganic acids, inorganic acid salts, acid anhydrides of an inorganic acid, or mixed anhydrides thereof either supported or unsupported. Specific catalysts mentioned include phosphoric acid, boric acid, silicic acid, titanic acid, boron phosphate, silicon borate, and silicon titanate.
British Pat. No. 2,093,060 describes the dehydration of carbonyl compounds to dienes using as catalysts magnesium phosphate compositions, while British Pat. No. 2,063,297 describes the use of alum and mixed alum sulfates as catalysts in the same reaction.
Zeolite molecular sieves have been used as catalysts for the production of isoprene by dehydration of starting materials other than 2-MBA. For example, Japanese Patent Application Publication No. 77/57104 describes the dehydration of 4,4-DMD in the gas phase over Group II metal-exchanged Zeolite X. Japanese Patent Application Publication No. 74/10924 describes the dehydration of 4,4-DMD or 4-methyl-5,6-dihydro-2H-pyran in the gas phase over a zeolite catalyst. Japan Patent Application Publication No. 77/36603 describes the dehydration of 4,4-DMD over a catalyst composed of Zeolite X and an acid clay. Nefedova et al., Neftekhimiya 19, 113 (1979) describe the dehydration of 2-methylbutane-2,3-diol using as catalysts Zeolites A, X, and Y. Ivanova and Kucherov, Neftekhimiya 10, 400 (1970) describe the use of Zeolite Y as a catalyst for the dehydration of dimethylvinyl- carbinol.
A variety of catalysts have also been proposed for use in the dehydration of 2-methylbutyraldehyde to isoprene. U.S. Pat. No. 1,033,180 describes the use of aluminum silicate as a catalyst. European Patent Application Publication No. 80,449 describes the use of acidic, heterogeneous catalysts, specifically boron phosphate and silica in this reaction. U.S. Pat. No. 4,524,233 describes the use of boron phosphate admixed with graphite and treated with amines and steam as a 2-MBA to isoprene catalyst. U.S.S.R. Pat. No. 721,116 and Bol'shakov et al., Neftekhimiya 23, 183 (1983) describe the use of boron phosphate as a catalyst for this reaction. Irodov, Smirnov and Kryukov, Zh. Org. Khim. 18, 1401 (1982) describe the use of amorphous aluminum, boron and calcium phosphates as catalysts for the conversion of 2-MBA to isoprene.
U.S. Pat. No. 4,560,822 to Hoelderich et al. (which issued December 24, 1985 on Application Ser. No. 730,687 filed May 3, 1985 and claiming priority of West German Patent Application No. 34 19 379 filed May 24, 1984) describes and claims a process for the dehydration of aldehydes to dienes (including the dehydration of 2-methylbutyraldehyde to isoprene) at elevated temperatures using a zeolite as a catalyst.
It has now been discovered that the dehydration of 2-methylbutyraldehyde to isoprene can be efficiently conducted using non-zeolitic molecular sieves as catalysts. The same catalysts can be used for the dehydration of other aldehydes to the corresponding dienes.